The present invention relates generally to wireless communication systems, and more particularly to decoding techniques for use in multiple-antenna wireless communication systems.
Multiple-antenna wireless communication links allow much higher data rates than traditional single antenna links. Under the assumption that the fading coefficients between the different antennas are independent and known to the receiver, it has been shown in, e.g., J. Foschini, xe2x80x9cLayered space-time architecture for wireless communication in a fading environment when using multi-element antennas,xe2x80x9d Bell Labs Tech. J., Vol. 1, No. 2, pp. 41-59, 1996, that the capacity of a multiple-antenna link scales linearly with the lesser of the number of send and receive antennas. Several techniques for modulation and coding over such channels have been proposed. See, e.g., V. Tarokh, N. Seshadri, and A. R. Calderbank, xe2x80x9cSpace-time codes for high data rate wireless communication: Performance criterion and code construction,xe2x80x9d IEEE Trans. Inf. Theory, vol. 44, pp. 744-765, 1998; and V. Tarokh, H. Jafarkani, and A. R. Calderbank, xe2x80x9cSpace-time block codes from orthogonal designs,xe2x80x9d IEEE Trans. Inf. Theory, vol. 45, pp. 1456-1467, July 1999.
In a mobile wireless environment, the fading coefficients change rapidly and the receiver may not have enough time to learn the fading coefficients. Even if the receiver does not know the fading coefficients, a substantial increase in channel capacity is still theoretically possible, as described in, e.g., T. L. Marzetta and B. M. Hochwald, xe2x80x9cCapacity of a mobile multiple-antenna communication link in Rayleigh flat fading,xe2x80x9d IEEE Trans. Inf. Theory, vol. 45, pp. 139-157, 1999. Improved capacity can be achieved in this unknown channel case using so-called unitary space-time signals, such as those described in U.S. patent application Ser. No. 09/206,843, filed Dec. 7, 1998 and entitled xe2x80x9cWireless Transmission Method for Antenna Arrays Using Unitary Space-Time Signals,xe2x80x9d which is incorporated by reference herein.
A well-known technique for dealing with unknown channels in a single antenna setting is differential modulation. Recently, a multiple antenna differential modulation technique using unitary space-time signals was introduced in U.S. patent application Ser. No. 09/356,387, filed Jul. 16, 1999 and entitled xe2x80x9cMethod for Wireless Differential Communication Using Multiple Transmitter Antennas,xe2x80x9d which is incorporated by reference herein. The framework is formally similar to standard one-antenna differential modulation and connects the known channel case with the unknown channel case. In particular, this Application introduces so-called diagonal codes which are simple to generate, e.g., every antenna transmits a PSK symbol in turn.
A differential multiple antenna scheme that builds upon the orthogonal designs of the above-cited V. Tarokh et al. reference has also been proposed. The advantage of these orthogonal design differential codes is their good performance, and the fact that fast decoding algorithms exist. A significant disadvantage is that they only exist for a restricted number of antennas, while diagonal codes exist for any number of antennas. Even though diagonal codes are simple to generate, only a slow decoding algorithm has heretofore been available for such codes. This algorithm is unduly complex in that it is exponential in both the number of antennas and the rate.
A need therefore exists for improved techniques for decoding in multiple-antenna wireless communication systems.
The present invention provides lattice-based techniques for the fast decoding of diagonal modulation in a multiple-antenna wireless communication system. In accordance with the invention, received symbols are decoded in a multiple-antenna communication system using lattice-based decoding. The symbols correspond to points in a modulation constellation, e.g., a diagonal modulation constellation, and at least a subset of the constellation is characterized as a lattice for decoding purposes. For example, if a given communication link of the multiple-antenna communication system includes M transmitter antennas and a single receiver antenna, the diagonal modulation constellation can be characterized as a lattice in M dimensions. The decoding operation for the received differential symbols involves a determination of the closest point in the lattice corresponding to the constellation. This determination may be made in an efficient manner using a basis reduction algorithm which generates an approximately orthogonal basis for the lattice, and then utilizes component-wise rounding to determine the closest point.
The invention thus approximately recasts the decoding problem into that of finding the closest point in an integer lattice corresponding to at least a subset of a modulation constellation, such that a conventional basis reduction algorithm, e.g., the so-called LLL algorithm, can then be used to solve the latter problem in an efficient manner.
The invention can be used in conjunction with the decoding of differential or non-differential modulation. For example, in an application in which channel characteristics are known or it is otherwise desirable to utilize non-differential modulation, the lattice-based decoding techniques of the invention may be applied to the decoding of the resulting non-differential symbols.
It should also be noted that the lattice-based decoding techniques of the invention may be applied to only a subset of a modulation constellation, rather than to an entire constellation. In this case, the subset of the constellation may be characterized as a lattice with decoding performed using the lattice-based techniques of the invention, while other portions of the constellation are decoding using other techniques.
The invention provides a number of significant advantages over conventional decoding for multiple-antenna systems. For example, unlike conventional maximum likelihood decoding, the complexity of which is exponential in both the number of antennas and the rate, lattice-based decoding in accordance with the invention has a complexity which is polynomial in the number of antennas and the rate. Moreover, it will be shown herein that the error rate performance of the lattice-base decoding of the present invention is very close to that of conventional maximum likelihood decoding.